The present invention relates to an insulation composite and, more particularly, a multilayer insulation composite.
A common type of insulation is multilayer insulation, which is especially useful for cryogenic applications. Multilayer insulation typically consists of alternating layers of highly reflecting material, such as aluminum foil or aluminized polyester (e.g., Mylar) film, and a low-conductivity spacer material or insulator, such as fiberglass mat or paper, glass fabric, or nylon net. Between twenty and forty such layers are commonly used for cryogenic applications including, for example, laboratory dewars, piping, on-site storage vessels, and transportation vessels (e.g., as part of tank trucks). In addition, multilayer insulation is advantageously kept under a high vacuum, thereby further enhancing the insulating properties of the multilayer insulation. Multilayer insulation has a very low heat transfer due to the fact that all modes of heat transferxe2x80x94conductive, convective, and radiativexe2x80x94are minimized. The multiple layers of reflecting material have a low emissivity and, thereby, inhibit radiative heat transfer. Convective heat transfer is inhibited by lowering the pressure (i.e., creating a vacuum) between the insulation layers. Finally, the presence of spacer material inhibits conductive heat transfer through thermal short-circuits (physical contact) that might otherwise exist between the layers of reflecting material.
Despite the satisfactory performance of conventional multilayer insulation composites in many applications, there remains a need for an improved multilayer insulation composite. The present invention seeks to provide such a multilayer insulation composite, particularly a multilayer insulation composite that provides satisfactory, if not superior, thermal performance, preferably with a reduced overall mass and/or thickness. These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.
The present invention is an insulating composite comprising (a) a first thermally reflective layer having a reflective surface and an opposite surface, (b) a second thermally reflective layer having a reflective surface and an opposite surface, and (c) a porous metal oxide film having a thickness of 20 xcexcm or less that is positioned between the first and second thermally reflective layers such that there is substantially no direct physical contact (thermal bridging) between the first and second thermally reflective layers. The present invention further includes an insulating element in which the insulating composite is disposed within an air-impermeable container.
The present invention provides an insulating composite having at least two thermally reflective layers with a porous metal oxide film positioned therebetween.
Any material that is effective in inhibiting radiative heat transfer can be used as the thermally reflective layer. Typically, such materials will have a reflective (e.g., polished) surface. The thermally reflective material preferably is characterized by a low emissivity. Also, the thermally reflective material typically will be in the form of a sheet or strip. Thus, the thermally reflective layer generally will have a reflective surface and an opposite surface. Suitable thermally reflective layers include, for example, aluminum foil. Other suitable thermally reflective layers include polymeric (e.g., polyester, polyamide, polyimide, or polyolefin) substrates having aluminum deposited on one or both surfaces thereof. Such a thermally reflective layer is commercially available as aluminized polyester (e.g., Mylar) film. Other thermally reflective materials having a low emissivity, such as gold and silver, can be deposited instead of aluminum on the aforementioned substrates in certain applications. The thermally reflective layer can have any suitable thickness, preferably about 10-100 xcexcm.
The thermally reflective layers can be the same or different. In particular, each thermally reflective layer can be constructed of the same or different material, and in the same or different manner, as other thermally reflective layers. In a preferred embodiment, all of the thermally reflective layers (e.g., the first and second thermally reflective layers) are aluminum foil, which is polished at least on one side.
The insulating composite of the present invention can further include additional thermally reflective layers, e.g., third, fourth, fifth, etc. thermally reflective layers. The discussion herein of the xe2x80x9cfirstxe2x80x9d and xe2x80x9csecondxe2x80x9d thermally reflective layers is equally applicable to these additional (e.g., xe2x80x9cthird,xe2x80x9d xe2x80x9cfourth,xe2x80x9d xe2x80x9cfifth,xe2x80x9d etc.) thermally reflective layers. Thus, the present inventive insulating composite can comprise successive layers of a thermally reflective material having a reflective surface and an opposite surface, such that the porous metal oxide film separates the layers of thermally reflective material. Because radiant-heat transfer is inversely proportional to the number of thermally reflective layers and directly proportional to the emissivity of these layers, radiant-heat transfer is minimized by using multiple layers of a low-emissivity thermally reflective material.
Any suitable porous metal oxide film (i.e., a porous continuous sheet or expanse of metal oxide) can be used in the insulating composite of the present invention consistent with ensuring that there is substantially no direct physical contact (thermal bridging) between the first and second thermally reflective layers, preferably between any of the thermally reflective layers, and that optimally there is no direct physical contact (thermal bridging) at all between the first and second thermally reflective layers, ideally between any of the thermally reflective layers. The porous metal oxide film most preferably is substantially coextensive or entirely coextensive with at least one of the first and second thermally reflective layers, and preferably, in some embodiments, both of the first and second (or even all of the) thermally reflective layers.
The porous metal oxide film can comprise any suitable type of metal oxide, such as, for example, silica, alumina, titania, zirconia, ceria, and magnesia. The metal oxide preferably is silica, such as, for example, fumed (or pyrogenic) silica, precipitated silica, silica aerogel, and silica xerogel, with fumed silica being particularly preferred. The metal oxide can be in the form of discrete individual particles, which can be in aggregated or non-aggregated form.
The porous metal oxide film can have any suitable density, typically about 2 g/cm3 or less (e.g., about 0.1-1.5 g/cm3), preferably about 1 g/cm3 or less (e.g., about 0.1-0.8 g/cm3), and most preferably about 0.7 g/cm3 or less (e.g., about 0.1-0.5 g/cm3). It is preferred that the porous metal oxide film have as low a density as possible inasmuch as lower densities generally provide improved thermal performance of the present inventive insulating composite.
The porous metal oxide film has a thickness of about 20 xcexcm or less, preferably about 10 xcexcm or less. The porous metal oxide film more preferably has a thickness of about 5 xcexcm or less, most preferably about 1 xcexcm or less, although typically at least about 200 nm (e.g., about 200 nm to about 1 xcexcm). It is preferred that the porous metal oxide film be as thin as possible inasmuch as thinner layers generally provide improved thermal performance of the present inventive insulating composite. The minimum spacing between the thermally reflective layers of the present inventive insulating composite is a function of the thickness of the porous metal oxide film (although, due to the nature of many thermally reflective layers which are not amenable to perfectly parallel spacing, the spacing between the layers typically will vary from the thickness of the porous metal oxide film to many times that thickness, e.g., up to about 500 xcexcm).
The porous metal oxide film generally will not be self-supporting and preferably is adhered to one or both sides of at least one thermally reflective layer by any suitable means. Alternatively, the porous metal oxide film can be adhered to a supporting substrate, e.g., a thin film (e.g., about 1-10 xcexcm thick) that is not a thermally reflective layer (such as a polymeric substrate, e.g., a polyester film), for ease of handling and positioning between the thermally reflective layers. In such an embodiment, the total thickness of the porous metal oxide film and supporting substrate preferably does not exceed the thickness values described above for the porous metal oxide film alone.
In one embodiment of the present invention, the insulating composite comprises first and second thermally reflective layers having reflective and opposite surfaces and a porous metal oxide film positioned therebetween which is adhered to the opposite surface of the first thermally reflective layer. In another embodiment, the opposite surface of the first thermally reflective layer faces the reflective surface of the second thermally reflective layer. In yet a further embodiment, the porous metal oxide film is adhered to the reflective surface of the second thermally reflective layer. A particularly preferred embodiment includes all of the features of the foregoing embodiments.
The porous metal oxide film can be prepared on and/or adhered to the surface of a thermally reflective layer by any suitable means. Preferably, the porous metal oxide film is deposited (e.g., electrostatically applied or dispersion coated), or prepared in situ, on the thermally reflective layer. The porous metal oxide film can be introduced from metal oxide in dry form (e.g., as a powder) or in a dispersion (e.g., with a carrier). For example, a dispersion of a metal oxide such as silica in a suitable carrier, such as water and/or alcohol, is deposited, e.g., by spraying or brushing, onto the substrate, preferably the thermally reflective layer, and then the carrier is evaporated, thereby leaving a porous metal oxide film. To ensure satisfactory formation of a metal oxide film, a suitable surfactant can be added to the metal oxide dispersion prior to deposition onto the substrate. The metal oxide dispersion also can contain an additive to control the pH of the metal oxide dispersion inasmuch as the pH can have an effect on the adhesiveness of the metal oxide particles to each other and the resulting density of the porous metal oxide film. A thicker porous metal oxide film can be formed by depositing additional metal oxide dispersion on top of the already formed porous metal oxide film and repeating the formation process any number of additional times, e.g., 2, 3, 4, or more times until the desired thickness of the porous metal oxide film is achieved.
The metal oxide utilized to form the porous metal oxide film can have any suitable physical characteristics. The metal oxide can be in the form of discrete individual particles, which can be in aggregated or non-aggregated form. The metal oxide can have any suitable discrete individual particle diameter, typically about 5 nm or more, and preferably about 5-20 nm. The metal oxide similarly can have any suitable aggregate particle diameter, typically about 500 nm or less. The metal oxide can have any suitable surface area, typically about 50 m2/g or more, preferably about 100 m2/g or more, and more preferably about 200 m2/g or more, or even about 300 m2/g or more. The surface area is calculated based on the amount of nitrogen adsorbed at five different relative pressures over the range 0.05 to 0.25 according to the Brunauer-Emmett-Teller (BET) model, which is referenced in Gregg, S. J. and Sing, K. S. W, xe2x80x9cAdsorption, Surface Area and Porosity,xe2x80x9d p. 285, Academic Press, New York (1991).
A suitable binder can be utilized in conjunction with the preparation of the porous metal oxide film, although its use is not preferred. If a binder is used, however, an inorganic binder is preferred, particularly sodium silicate. The binder can be used in a variety of manners. For example, the binder can be applied to one of the surfaces of the thermally reflective layer (preferably the surface opposite the reflective surface thereof), with the metal oxide dispersion being subsequently deposited onto the binder. Alternatively, the binder can be mixed into the metal oxide dispersion, and then the mixture can be deposited onto the surface of the thermally reflective layer.
Other additives can be present in the insulating composite of the present invention. For example, hydrogen and water getters can be used. As with the porous metal oxide film, these additional components can be either loosely present in the insulating composite between one or more thermally reflective layers or adhered to one or more thermally reflective layers. One suitable method of incorporating such additives into the present inventive insulating composite is to mix the additives with the metal oxide dispersion, which then can be applied to the surface of one or more thermally reflective layers.
In order to facilitate the handling of the present inventive insulating composite, particularly when many layers are utilized, and/or to provide a measure of protection to the present inventive insulating composite, an outer or cover layer can be used in conjunction with the present inventive insulating composite. Such an outer or cover layer is optional.
The insulating composite of the present invention can be used to insulate any suitable item. The insulating composite of the present invention can be applied directly to the surface for which insulation is desired. For example, the insulating composite can be placed directly on the outer surface of dewars, piping, storage vessels, and transportation vessels (e.g., the tanks of tank trucks). Multiple sheets or strips of thermally reflective layers can be applied to surfaces to produce a multilayer insulating composite in accordance with the present invention. Alternatively, sheets or strips of the present inventive insulating composite can be wrapped around the surface to be insulated, such as pipes or tanks. Separate individual sheets or strips can be used to-wrap surfaces or one continuous sheet or strip can be wound around surfaces, for example, such that the insulating composite has a spiral-shaped configuration.
Depending upon the particular application, the reflective surface of the thermally reflective layer can face toward or away from the surface to be insulated. In insulating composites having more than one thermally reflective layer, all of the reflective surfaces preferably face the same direction. In applications where it is desired to keep a surface at a cooler temperature than the surrounding environment (e.g., cryogenic applications), the reflective surfaces preferably face away from the cooler surface. This configuration minimizes radiative heat transfer from the warmer environment to the cooler surface. Alternatively, in applications where it is desired to keep a surface at a warmer temperature than the surrounding environment (e.g., process piping for hot fluids), the reflective surface preferably face toward the warmer surface. This configuration minimizes radiative heat transfer from the warmer surface to the cooler environment.
The insulating composite of the present invention can be disposed within a container to form an insulating element. The container can be any suitable container, for example, a container prepared from a polymeric material. In some embodiments, the container desirably is an air-impermeable container. The insulating element containing the insulating composite can be utilized in the same manner as the insulating composite itself, e.g., applied to the outer surface of an item requiring insulation, e.g., a storage vessel. The container also can have suitable dimensions to contain both the insulating composite and the item requiring insulation itself, e.g., a storage vessel.
The use of an air-impermeable container permits the insulating composite disposed within to be subjected to a pressure below atmospheric pressure, which results in the reduction of heat transfer, by conduction. The lower pressure is attained by establishing a vacuum within the air-impermeable container, which can be done using conventional vacuum equipment. The pressure within the air-impermeable container preferably is about 10xe2x88x921 kPa (1 torr) or less, more preferably about 10xe2x88x922 kPa (10xe2x88x921 torr) or less, and most preferably about 10xe2x88x924 kPa (10xe2x88x923 torr) or less. Optimally, the pressure within the air-impermeable container is even lower, e.g., about 10xe2x88x926 kPa (10xe2x88x925 torr) or less, or even about 10xe2x88x928 kPa (10xe2x88x927 torr) or less.
The insulating composite and insulating element of the present invention are suitable for use as insulators in a wide variety of applications over a wide variety of temperatures. For example, the inventive insulating composite can be used for cryogenic applications such as laboratory dewars, process piping, storage vessels, and transportation vessels. Additionally, the insulating composites disclosed herein can be used in superconducting magnet applications as disclosed in U.S. Pat. No. 5,143,770. In addition to cryogenic applications, insulating composites and insulating elements of the present invention can be used in extremely high temperature applications such as, for example, aerospace vehicles, firemen""s suits, and industrial tools.